The present invention relates to telecommunications systems in general, and in particular to the transmission of compressed signals in telecommunications system.
Telecommunications has moved in the recent years towards digital networks carrying voice, facsimile and other signals. One known way in the art to increase the efficiency of such networks is to transmit the signals in a compressed form, thus using the available bandwidth for simultaneous transmission of more information.
U.S. Pat. No. RE 35,740 discloses a system for carrying voice, facsimile and non-facsimile voice band data. This system includes various compressing mechanisms and offers an improved detection of facsimile signals. Still, when speech, facsimile and other signals are subjected to several compression/decompression cycles, their quality degrades substantially, and at times, the signals may be received at a quality and delay that are unacceptable.
Some methods were suggested in the past to overcome this handicap. Cox et al., in International Conference on Communications, Vol. 1, June 1988, pp. 90-95, suggest a method to decrease the distortion and delay introduced while using low rate speech coding. It was suggested there that the receiving module, the compressor, searches the less significant bits of the input 64 Kb/s mu-law coded stream for the synchronization pattern. If it detects the pattern, it temporarily squelches the compressed signal and enables the de-compressor to begin padding rather than decoding. When the de-compressor is padding the low-rate frame, a new synchronization pattern is inserted into one of the non-used bits. When the far end compressor detects this new synchronization pattern it disables the squelch on its compressor and begins stripping the padded bits. However, when the compressor at the receiving end does not find the xe2x80x9crobbedxe2x80x9d less significant bits synchronization pattern in the 64 Kb/s stream, it will not proceed to the pad and strip state, and the de-compressor will insert the robbed bit synchronization pattern into the 64 Kb/s stream, going to the subscriber. One way of increasing the rate of the low rate bit stream suggested in this publication, is by inserting the low rate signal into the least significant bits of the mu-low word of the 64 Kb/s stream. The advantage of using such a method is that if by any chance the system treats the signal as mu-low coded speech, only low level noise signal would be noticed by the human listener.
However, one of the main drawbacks of the method described, is the fact that no other signals than voice signals can be transmitted along a transmission path comprising a number of operative compressing/decompressing devices. Furthermore, none of the methods described discloses a solution for the problem of handling a change in the type of the transmitted signals, while retaining the continuity of the communication session (e.g. switching from voice to fax during a communication session). Moreover, the traffic congestion in telecommunication networks is of a varying nature, and obviously for a system to operate efficiently it should also be adaptable to such variations.
It is therefore an object of the present invention to provide a digital telecommunication station capable of efficiently transmitting signals originating from transmitting sources of various types.
It is another object of the present invention to provide a digital communication system utilizing efficiently the bandwidth available for communication transmissions while retaining a good quality of the signal and session continuity.
Yet another object of the present invention is to provide a method for efficient handling of various types of signals.
Further objects and features of the invention will become apparent to those skilled in the art, from the following description and the accompanying drawings.
In accordance with the present invention there is provided a digital telecommunication station adapted to receive at least two different types of signals, operative in a digital communication system, and comprising:
at least one detector operative to receive signals carried along a transmission path and determine their type;
at least one switch controlled by said at least one detector, adapted to channel said signals in accordance with the determination of their type;
first identifier for determining whether said signals received are of a digital compressed form;
second identifier for determining whether the transmission path includes at least one further operative de-compressor adapted for decompressing signals when being transmitted in their compressed form;
at least two pairs of compressing/decompressing devices, each adapted to compress/decompress a different type of signals;
said transmission path does not include at least one fur first transmitter operative in response to a determination made by said second identifier that ther operative decompressor adapted for decompressing the signals being transmitted in their compressed form; and
second transmitter operative in response to a determination made by said second indentifier that said transmission path does include at least one further operative decompressor adapted for decompressing the signals being transmitted in their compressed form into the decompressed digital output signals.
Preferably, said at least two different types of signals are selected from the group consisting of voice signals, fax signals, data signals, voiceband data signals and video signals. More preferably, the types of signals received by the digital communication station of the present invention include at least voice and fax types of signals.
According an embodiment of the present invention, the pairs of different compressing/decompressing devices are preferably selected from the group comprising:
encoder/decoder adapted to receive a voice signal and operate thereon;
modulator/demodulator adapted to receive a facsimile signal and operate thereon;
encoder/decoder adapted to receive a data signal and operate thereon;
voice-band data modulator/demodulator adapted to receive a voice band data signal and operate thereon; and
video encoder/decoder adapted to receive a video signal and operate thereon.
By a preferred embodiment, the digital telecommunication station of the present invention is adapted to receive a signal classification message denoting the type of digital signals received by that digital telecommunication station. More preferably, the signal classification message is transmitted to denote transmission of digital signals of a type, which differs from the type of their preceding digital signals transmitted by the station during a communication session. Typically, a signal classification message is a message that is included in a frame, block or the like, and characterizes the type of digital signals included in that same frame. Preferably, the signal classification message is included in a frame carrying digital signals sand precedes the compressed digital signals of a new type contained in that frame. However, as would be appreciated by a person skilled in the art, in systems which can tolerate delays in transmission, it would also be possible to have the signal classification message indicate that digital signals of a new type have already been transmitted. Furthermore, the signal classification message may denote a change in the signal type occurring in a neighboring frame (the one before or the one after the frame which includes that message) or even in non-neighboring frames. Of course, when the change occurs in a non-neighboring frame to that which comprises the signal classification message, the message should preferably also direct to the frame in which the new type of signals are introduced for the first time, e.g. by providing an indication to the number of frames separating between these two frames, etc.
Embedding in a frame the classification of the type of signals to follow has significant advantages that the present invention provides over the known art. One of the major advantages is that in an environment that includes compressing devices, this way of operation allows to change the type of signals being transmitted during a communication session (e.g. switching from fax to voice or vice versa) while retaining the session continuity.
According to the above embodiment of present invention, a signal classification message may either be carried through the telecommunication station when generated upstream of the station, or be generated by the station and transmitted towards a further downstream telecommunication station.
A typical such signal classification message identifying the type of the signals to follow comprises the information signals of the type identified in their compressed form and stuffing bits filling the remainder of the message. According to a preferred embodiment of the invention, the stuffing bits in a message may be signals that represent the non-compressed form of the message""s compressed information signals or a part thereof. Using such non-compressed signals allows reasonable monitoring of the communication session and detecting special signaling tones such as dual tone, etc.
According to a preferred embodiment of the invention, in the initialization stage of a communication session in a telecommunication station, a pre-selected type of signal is assumed, e.g. that session will be initialized as a session of voice transmission. The pre-selected type of signals would remain valid until a signal classification message identifying a new type of signal is received at the station. Preferably, the pre-selection of the signals"" type is made by the system operator, reflecting the type of the most commonly expected type of traffic in the specific communication network. By one alternative of the invention, the signal classification message is embedded only prior to the occurrence of an event where an actual change the type of signals takes place. By another alternative, the signal classification message is transmitted prior to a first change in the signal type and is included thereafter in all consecutive messages, irrespective of whether there is a change in the signal type or not, until the termination of that communication session.
According to another embodiment of the invention there is provided a digital communication system for interconnecting a plurality of telecommunication trunks, carrying at least two different types of signals, via a transmission path, comprising:
first transmission means at least a first end of the transmission network for transmitting digital signals;
at least one pair of telecommunication stations of the type specified;
at least one further operative means adapted for decompressing signals being transmitted in their compressed form; and
receiving means at at least a second end of the transmission network.
The term xe2x80x9ctelecommunication networkxe2x80x9d as will be used hereinafter, should be understood to encompass the various types of networks known in the art, such as TDM, synchronous and asynchronous transfer networks, IP networks, IP frame relaying networks and any other applicable packet communication networks.
As previously explained, the term xe2x80x9ctelecommunication stationxe2x80x9d is used herein to describe a combination of at least two compressing/decompressing devices, one of which is used for compressing signals when required, while the other is used as its corresponding de-compressor (e.g. one such device may be an encoder while the other a decoder, etc.). These two devices may be included within one apparatus or be separated from each other.
A xe2x80x9cnormal compression modexe2x80x9d of a pair of such telecommunication stations as will be referred to hereinafter, is used to denote a mode of operation in which the incoming digital signals received by a first station are compressed by the station""s compressing device (the compressor), transmitted in a compressed form to the de-compressing device (decompressor) of the second station of that pair of telecommunication stations, where decompression is carried out essentially by converting the digital signals into their non-compressed form.
In a case where there is at least one further operative means adapted to decompress signals being transmitted in their compressed form, e.g. in a further pair of telecommunication stations located downstream along the transmission path, the mode of operation will be somewhat different. The information signals received will again be compressed by the compressor of the first telecommunication station, and their compressed form will be transmitted to the de-compressor of the second station of the first pair of stations. However, in this case, the compressed message will not be decompressed by the de-compressor of that second station, but instead, will be transmitted in its compressed form towards that at least one further operative means. In the case that there is a further operative pair of telecommunication stations, the message may be transmitted via the compressor of the first station of the second pair to the de-compressor of the second station of that second pair where it will be decompressed essentially into the non-compressed form of the digital signals. Still, the decompression will be carried out provided that the determination made by the identification means of that second station of the second pair of telecommunication stations is, that there is no other operative decompressor downstream. The decompressed message will then be transmitted by the first station transmission means of the second station of the second pair of telecommunication stations towards the receiving means at the downstream end of the transmission path. Such a mode of operation will be referred to hereinafter as xe2x80x9cend-to-end compressionxe2x80x9d.
According to yet another preferred embodiment of the invention, the signal classification message characterizing the type of the following signals further comprises information that allows synchronizing the operation of the telecommunication stations operating along the transmission path. For example, the signal classification message may further include an indication of number of bits, which enables the receiving station to identify precisely when the transmission of information signals starts.
In accordance with still another preferred embodiment of the invention the signal classification message further comprises an indication of the bit integrity. Examples of such indications are check sum, various CRCs, and the like.
According to further preferred embodiment of the present invention. The transmission network of the invention further comprises means for determining the mode of operation of facsimile signals. More preferably, the first transmission means also comprises means for transmitting over the transmission network and to the receiving means, an indication of the mode of operation of the facsimile signals.
In accordance with yet another preferred embodiment of the invention, one or more of the pairs of telecommunication stations in the communication system may be operated selectively.
According to another embodiment of the present invention, the system further includes message generation and message detection means where the latter means is operative at the receiving end to detect messages transmitted by the first transmitting means.
By a further embodiment of the present invention, at least one of the digital telecommunication stations in the digital communication system is further provided with an option of establishing a communication link with more than two other digital telecommunication stations. Preferably, such a station is adapted to establish a communication with a plurality of digital telecommunication stations, each located at a different end of a telecommunication network. By another embodiment of the present invention, a full or part of a transmission sent by at least one of the telecommunication stations in the digital communication system may be received in parallel by at least two different digital telecommunication stations.
As previously explained, the prior art is concerned with the case where the signals transmitted in the xe2x80x9cend-to-end compressionxe2x80x9d mode, are voice signals. Sacrificing less significant bits in voice carrying messages, does not constitute a major problem, as the worst case that is likely to happen is that the noise level of the conversation is increased by a certain extent. However, trying to apply this method to communication sessions in which non-voice signals are transmitted, does create a problem, for which no solution has previously been presented. Once some of the bits communication are sacrificed, the result may be a failure in communication, as the receiving end can no longer recognize the transmission thus received.
According to yet-another embodiment of the present invention there is provided a method for use in a digital communication system that comprises at least a first and a second telecommunication stations and adapted to transmit at least two different types of signals, which method comprises:
(i) transmitting a message comprising a sequence of characters identifying a first transmitting means, the type of signals to be transmitted and their destination;
(ii) establishing a communication link between a first transmitting means and a receiving means at the transmission destination;
(iii) exchanging signals between a first pair of telecommunication stations, identifying for each one of them the existence of the other and the rate for transmitting signals therebetween;
(iv) transmitting a message comprising a characterization of the signals to follow;
(v) transmitting information signals at the rate defined;
(vi) transmitting a message denoting the end of transmission; and
(vii) disconnecting the communication link between the first transmitting means and the receiving means.
As previously described, the transmission path may include more thank two pairs of telecommunications stations, in which case a similar procedure of xe2x80x9chand-shakingxe2x80x9d as described in step (iii) above, will be carried out for each two pairs of consecutive stations.
According to a preferred embodiment of the invention, the method provided is used for interconnecting a plurality of telecommunication trunks carrying at least two different types of signals.
By a preferred embodiment of the method provided by the invention, a periodic check is conducted to confirm that at least one further means adapted for decompressing signals being transmitted in their compressed form is still operative along the transmission path. Therefore, when a station becomes the last decompressing operative means along that path, it will immediately resume its de-compression mode, and the pair of telecommunication stations to which this station belongs, will begin operating under normal compressing mode. According to a further embodiment of the present invention, prior to disconnecting an operative decompressing means from the transmission path to which it is connected, a message declaring the expected disconnection of that operative compressing means is sent. When such a message is received by the neighboring telecommunication stations, each on of them will immediately resume its normal compression mode. According to a more preferred embodiment of the invention, all pairs of telecommunication stations operating along the transmission path periodically transmit their integrity signals. Once a pair of stations does not receive the integrity signal of its neighboring downstream pair of stations, it will automatically resume thee normal compression mode.
According to the end-to-end compression mode of operation described in the art, when voice signals are received at a downstream pair of telecommunication stations in a compressed form, the station operating as the encoder is disabled, and the compressed information is transmitted to the station operating as a decoder, either for decompressing if there is no further pair of telecommunication stations downstream, or for transmission to a further pair of telecommunication stations operative downstream, in a compressed form. In accordance with yet another aspect of the present invention, the telecommunication station operating as the compressor of a pair of downstream digital telecommunication stations, is not disabled as suggested in the art, but is used to keep track of the compression parameters in the messages being transmitted. Consequently, when normal compression operation is to be resumed (e.g. when a downstream pair of telecommunication stations is disconnected from the transmission path), the compressing station may revert rapidly to its normal compression mode by using the correct and updated internal compression parameters.